MODES OF PRODUCTION OF A LIQUID MIST BETWEEN ELECTRODES
A gas can be purified highly efficiently by dispersing a liquid reactant between the electrodes of an electrostatic filter. Several methods have been employed, proposed or patented in order to produce a liquid mist in this type of contactor between three media, respectively gaseous, liquid and solid:
1) electrostatic nebulizing, at the asperities of the high-voltage electrodes, of a liquid coming from a reservoir with pressure head and electric charge (French Patent No. 1,406,086 of May 06, 1964) PA1 2) electrostatic nebulizing, at the asperities of the electrodes at earth potential, of a liquid supplied through pipelines at the top of these electrodes PA1 3) (primary) spraying, by means of mechanical, pneumatic or hydraulically pressurized atomizers at earth potential (U.S. Pat. No. 2,874,802 of Feb. 24, 1959 and French Patent No. 73,18584 of 22, May 1973) PA1 4) electrostatic nebulizing, at the asperities of the high-voltage electrodes, of the liquid runoff supplied by primary spraying using atomizers at the earth potential (French Patent No. 74,17094 of 16, May 1974) PA1 5) "to-and -fro" electrostatic nebulizing, at the asperities carried by two families of electrodes, respectively at high voltage and at earth potential (U.S. Pat. No. 3,785,118 of Jan. 15, 1974. This arrangement, called "bi-ionized field" promotes the agglomeration of the particles in suspension, to the detriment of their electrical sedimentation. It is not in general desirable because it promotes the return of sludge into suspension in the gas. PA1 1) washing of the collector electrodes when the deposition cannot be removed by mechanical means; PA1 2) decrease of the temperature of the gases treated, and consequently reduction of their volume flowrate through the electrostatic filter; PA1 3) agglomeration of the dust via the liquid drops either by effect of impact or by mutual electrostatic attraction; PA1 4) absorption and chemical treatment of the gaseous constituents capable of participating in the electrochemical corrosion of the metal structures; PA1 5) ionic conduction by the liquid film when the structures of the electrostatic filter are not electronically conducting. This is the case, for example, when the casing is made of masonry internally lined with a coating and when the electrodes are plates of tubes or polymer materials in order to avoid electrochemical corrosion of the apparatus; PA1 6) removal of the harmful gases such as: HCL, HF, SO.sub.2, NH.sub.3, NO.sub.x, odours, etc., the wet electrostatic filter then acting as a bi- or triphase reactor. PA1 1) they produce transfer of the polluting substances from a gaseous effluent to a liquid effluent, and therefore solve a problem only by creating another; PA1 2) their high cost is a deterrent as long as the pollution standards remain a weak constraint and monitoring of the industrial plant unstrict; PA1 3) the consumption of the washing liquid, of water in general, is high and often incompatible with the local supply possibilities; PA1 4) the technology of the apparatuses proposed takes into account neither the specificity of certain reactants used in spraying, for the requirement for depolluting the liquid effluent. PA1 1) it is now obligatory for any potential polluter henceforth to conform with much more restrictive European standards, and consequently to invest in higher-performance apparatuses; PA1 2) consequently, the opportunity for equipment suppliers to invest in research with a view to improving the techniques which are currently most promising, without being held back by financial constraints as severe as in the past; PA1 3) the current research, investment and progress in the matter of treatment of water, which contribute to minimizing the drawbacks linked with the transfer of pollution; PA1 4) the design of a "contactor with multiple stages and with counterflow between a gas and a liquid mist", exploited in the present patent in the form of an electrostatic reactor between three states, respectively liquid, solid and gaseous, constitutes a suitable technological solution to the problem of physico-chemical treatment of gaseous and liquid media within one and the same method. PA1 a) It purifies a gas and simultaneously concentrates the transfer liquid or liquids in a contact method with multiple stages and gas/liquid counterflow (liquid back-flow). Since the pollutant transfers take place within aerosols, the back-flow is necessarily produced from the liquid contained in the accumulation tanks which collect, via funnels, the run-off of the planar or tubular electrodes of the electrostatic filter/washer and partly recycles it into the corresponding spraying fields; PA1 b) It combines with the gas treatment line a line for treatment of the liquids withdrawn at the particular accumulation tanks called "extraction tanks", with a view, on the one hand, to completely or partially remove the undesirable constituents by suitable separation techniques and, on the other hand, to recycle, at the particular stages, a partially or completely purified process liquid, and optionally to return into the gas treatment line regenerated reactants or liquid or gaseous residues coming from the treatment of the liquids and the sludge. PA1 a) The flow rate, the composition and the spatial distribution of the primary mist can be adapted, at each spray field, to the local and temporal characteristics of the gas stream (temperature, humidity, chemical composition of the gases, continuous or discontinuous emission regime); PA1 b) It is possible to produce a continuous liquid film at the surface of the collector and emitting electrodes, avoiding, on the one hand, an excess run-off which are responsible for excessively frequent short-circuits by the uninterrupted liquid crickle between the bottom of a high-voltage electrode and the casing and, on the other hand, regions of drying responsible for local burning of the electrodes when they are manufactured from an electrically insulating organic material. PA1 a) The possibility of subjecting the gas to successive treatments, on line, by liquids of different compositions, which constitutes one of the innovations of the apparatus, response to the concern of treating the gas which have been most heavily loaded using the least expensive reactants, of adapting the composition of the liquid reactant to the local and temporal composition of the gas and of reserving the last spray fields for the use of highly specific reactants for the transfer of certain residual gaseous pollutants; PA1 b) The concentration of the pollutant as far as the extraction tanks, obtained by acting simultaneously on two mechanisms, on the one hand recycling the spray liquid at one and the same field of funnels and, on the other hand, the multiple-stage back-flow produced by transport of the liquid from one accumulation tank to the next, which also constitutes an innovation of the apparatus, makes it possible to optimize the specific treat-ments of the gas and those of the liquids with a view to removing the undesirable products in the form either of solids or of upgradable concentrated solutions. The back-flow of liquid may follow two paths, that of the direct transport from one accumulation tank to the next, or that which consists in taking off the liquid from one accumulation tank in order to carry out continuous or discontinuous partial additional spraying flowing into the adjacent accumulation tank.